1. Field of the Disclosure
This disclosure relates generally to drilling of deviated wells into earth formations, and more particularly to using earth models built in real time for maintaining the drilling in a desired direction.
2. Description of the Related Art
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
Traditionally, the well planner is given a starting point, and an indication of possible dog leg severity, and some targets. With these indications, the well planner designs a trajectory that fulfils the requirement. However, most of the time, the planning does not take into account the geometry of the geological layers that may be traversed. A definite improvement of well planning is made when the well planner process uses a real-time earth model that can be used in planning the trajectory so as to increase the exposure to the reservoir. This allows for rapid changes of trajectory, better reservoir exposure and maximise wellbore life span.
In development of reservoirs, it is common to drill boreholes at a specified distance from fluid contacts within the reservoir. An example of this is shown in FIG. 2. A resistivity sensor 119 on a bottomhole assembly 121 may be used for making resistivity measurements. A drill-bit indicated by 117 drills the borehole 115 where a porous formation denoted by 105a, 105b has an oil water contact denoted by 113. The porous formation is typically capped by a caprock such as 103 that is impermeable and may further have a non-porous interval denoted by 109 underneath. The oil-water contact is denoted by 113 with oil above the contact and water below the contact: this relative positioning occurs due to the fact the oil has a lower density than water. In reality, there may not be a sharp demarcation defining the oil-water contact; instead, there may be a transition zone with a change from high oil saturation at the top to a high water saturation at the bottom. In other situations, it may be desirable to maintain a desired spacing from a gas-oil. This is depicted by 114 in FIG. 1. It should also be noted that a boundary such as 114 could, in other situations, be a gas-water contact.
The use of real-time measurements has a definite benefit in reservoir navigation. When receiving data from downhole measurement tools, these data are presented as curves following a line, either vertical or horizontal, that represents the well-bore trajectory. These representations do not take into account the changes in azimuth or inclination of the actual wellbore trajectory. The present disclosure uses a 3-D representation of the data as an aid to interpretation.
The use of Azimuthal Propagation Resistivity (APR) measurements is a good example of how the earth model helps in reservoir navigation; APR is by definition an azimuthal measurement that is best represented in a 3-D space than on a 2-D plot. The outcome of these measurements is a distance and an azimuth corresponding to the location of the closest resistivity contrast detection. This resistivity contrast may be interpreted as a geologic layer interface or a fluid interface. Therefore APR measurements display where the closest bed boundary is. An earth model built in real-time provides a contextual meaning for these measurements because the earth model is a stack of bed boundaries. The measurement can therefore be easily interpreted. It is possible to define a surface passing through several detected beds along the wellbore trajectory. The layer dips and azimuths can then be incorporated into the earth model to reflect the APR measurements.